1. Field of the Invention
The present invention relates to an augmentation-index measuring apparatus that measures an augmentation index of a living subject, based on a cuff pulse wave obtained from a cuff that is worn on a portion of the subject""s body to press the body portion.
2. Related Art Statement
Augmentation index, generally known as AI, indicates, e.g., a proportion of a reflected-wave component of a pulse wave to an incident-wave component of the same, and is used to evaluate compliance of aorta. As the compliance of aorta increases, the reflected-wave component decreases and, as the compliance of aorta decreases, the reflected-wave component increases. More specifically described, as wall of aorta hardens, a reflected-wave component contained in waveform of pulse wave obtained from the aorta increases. Thus, augmentation index reflects arteriosclerosis, and can be used as an index for inspecting arteriosclerosis.
As described above, augmentation index indicates a proportion of a reflected-wave component of a pulse wave to an incident-wave component of the same, but it is difficult to separate a pulse wave detected (hereinafter, referred to as a detected pulse wave) into its incident-wave and reflected-wave components. Hence, an augmentation index may be determined as follows: First, a detected pulse wave is analyzed to identify respective peak points of incident-wave and reflected-wave components of the pulse wave. Then, the augmentation index is calculated by dividing a difference between a magnitude of the detected pulse wave at the time of occurrence of the peak of the incident-wave component and a magnitude of the pulse wave at the time of occurrence of the peak of the reflected-wave component, by a pulse pressure of the pulse wave. In addition, the peak of the incident-wave component may be determined as an inflection point or a local-maximum point between a rising point of the detected pulse wave and a peak of the same; and the peak of the reflected-wave component may be determined as the first local maximum point following the peak of the incident-wave component.
Since augmentation index is used to evaluate compliance of aorta as described above, it is a clinical practice to non-invasively detect a pulse wave from a carotid artery that is, of arteries located in the vicinity of body surface, the nearest to the aorta, and determine an augmentation index based on the carotid pulse wave. However, first, it needs adequate skill to wear, at an appropriate position, a carotid-pulse-wave sensor for detecting a carotid pulse wave and, second, it is needed to use or employ the carotid-pulse-wave sensor. Thus, there has been a need to easily measure an augmentation index using a cuff pulse wave detected from a cuff that is worn on, e.g., an upper arm of a living subject for measuring a blood pressure of the subject.
However, since a cuff pulse wave is detected as an oscillation of pressure in a cuff, change of the cuff pulse wave is considerably moderate, and accordingly there are many cases where it is difficult to identify respective peak points of incident-wave and reflected-wave components contained in the cuff pulse wave. In addition, there are some cases where the peak of the reflected-wave component is not the first local maximum point following the peak of the incident-wave component. In those cases, an accurate augmentation index cannot be determined based on respective magnitudes of the cuff pulse wave at the respective times of occurrence of the respective peaks of the incident-wave and reflected-wave components.
It is therefore an object of the present invention to provide an augmentation-index measuring apparatus that can measure an accurate augmentation index based on a cuff pulse wave.
The Inventors have carried out extensive studies and found that though respective peak points of incident-wave and reflected-wave components contained in a cuff pulse wave may not be clearly identified from the waveform of cuff pulse wave, there is a phenomenon that zero-crossing points of a fourth-order-differentiated waveform of the cuff pulse wave advantageously correspond to the respective peak points of incident-wave and reflected-wave components, and that an accurate augmentation index can be determined as a ratio of one of respective magnitudes of the cuff pulse wave at the respective peak points of the incident-wave and reflected-wave components determined based on the zero-crossing points of the fourth-order-differentiated waveform, to the other. The present invention has been developed based on this finding.
The above object has been achieved by the present invention. According to the present invention, there is provided an augmentation-index measuring apparatus comprising a cuff which is adapted to be worn on a portion of a living subject to press the portion; a fourth-order-differentiating device or means for subjecting a cuff pulse wave obtained from the cuff, to a fourth-order differentiation, and thereby obtaining a fourth-order-differentiated waveform; a peak-point determining device or means for determining, based on the fourth-order-differentiated waveform obtained by the fourth-order-differentiating device, a peak point of an incident-wave component contained in the cuff pulse wave and a peak point of a reflected-wave component contained in the cuff pulse wave; and an augmentation-index determining device or means for determining an augmentation index of the subject that indicates a proportion of one of a first magnitude of the cuff pulse wave at a time of occurrence of the peak point of the incident-wave component and a second magnitude of the cuff pulse wave at a time of occurrence of the peak point of the reflected-wave component, to the other of the first and second magnitudes.
According to the present invention, the peak-point determining device determines, based on the fourth-order-differentiated waveform obtained by the fourth-order-differentiating device, the peak point of the incident-wave component contained in the cuff pulse wave and the peak point of the reflected-wave component contained in the cuff pulse wave, and the augmentation-index determining device accurately determines the augmentation index indicating the proportion of one of the magnitude of the cuff pulse wave at the time of occurrence of the peak point of the incident-wave component and the magnitude of the cuff pulse wave at the time of occurrence of the peak point of the reflected-wave component, to the other.
According to a preferred feature of the present invention, the augmentation-index measuring apparatus further comprises a reference-point determining device or means for determining, as a reference point, a rising point of the cuff pulse wave; and a window determining device or means for determining a rising-point window based on the rising point determined by the reference-point determining device, and the peak-point determining device determines the peak point of the incident-wave component, based on a zero-crossing point of the fourth-order-differentiated waveform that falls in the rising-point window determined by the window determining device. Thus, the peak point of the incident-wave component is more accurately determined and the augmentation index is more accurately determined based on the peak point, as compared with a case where a peak point is determined on a moderate waveform.
According to another feature of the present invention, the peak-point determining device selects, as the peak point of the incident-wave component, one of a plurality of zero-crossing points of the fourth-order-differentiated waveform that fall in the rising-point window, such that the selected one zero-crossing has a predetermined relationship with respect to a starting point or an ending point of the rising-point window. According to this feature, since the zero-crossing point having the predetermined position is determined as the peak point of the incident-wave component, the peak point of the incident-wave component is more accurately determined and the augmentation index is more accurately determined based on the peak point.
According to another feature of the present invention, the augmentation-index measuring apparatus further comprises a reference-point determining device or means for determining, as a reference point, a notch point of the cuff pulse wave; and a window determining device or means for determining a notch-point window based on the notch point determined by the reference-point determining device, and the peak-point determining device determines the peak point of the reflected-wave component, based on a zero-crossing point of the fourth-order-differentiated waveform that falls in the notch-point window determined by the window determining device. Thus, the peak point of the reflected-wave component is more accurately determined and the augmentation index is more accurately determined based on the peak point, as compared with a case where a peak point is determined on a moderate waveform.
According to another feature of the present invention, the peak-point determining device selects, as the peak point of the incident-wave component, one of a plurality of zero-crossing points of the fourth-order-differentiated waveform that falls in the notch-point window, such that the selected one zero-crossing has a predetermined relationship with respect to a starting point or an ending point of the notch-point window. According to this feature, since the zero-crossing point having the predetermined position is determined as the peak point of the reflected-wave component, the peak point of the reflected-wave component is more accurately determined and the augmentation index is more accurately determined based on the peak point.
According to another feature of the present invention, the augmentation-index measuring apparatus further comprises a cuff-pressure changing device which changes, for a blood-pressure measurement, a pressure in the cuff from a pressure higher than a systolic blood pressure of said portion of the subject to a pressure lower than a diastolic blood pressure of said portion of the subject, and the cuff pulse wave is obtained, before or after the blood-pressure measurement, from the cuff having a pressure lower than the diastolic blood pressure of the subject obtained from the blood-pressure measurement. According to this feature, the augmentation index can be obtained simultaneously with the blood-pressure measurement.
According to another feature of the present invention, the augmentation-index measuring apparatus is used as an arteriosclerosis inspecting apparatus. That is, the arteriosclerosis inspecting apparatus is used for inspecting arteriosclerosis of the subject based on the augmentation index determined by the augmentation-index determining device.